Geochemistry of Eastern Pacific Morb: implications for morb petrogenesis and the nature of crustal accretion within the neovolcanic zone of two recently active ridge segments

172 Pages·1999·6.4 MB·English

by  Smith, Matthew C

GEOCHEMISTRYOFEASTERNPACIFICMORE:
IMPLICATIONSFORMOREPETROGENESISANDTHENATUREOFCRUSTAL
ACCRETIONWITHINTHENEOVOLCANICZONEOFTWORECENTLYACTIVE
RIDGESEGMENTS
By
MATTHEWC.SMITH
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1999
ACKNOWLEDGMENTS
Therearemanythathavecontributedtothisresearchanddeserve
aclcnowledgement. IwouldliketothankDr.MichaelPerfitforhelpingtoputmeina
positiontotakeadvantageofnumerousopportunitiestoparticipateonthefascinating
oceanographiccruisesthathaveledtothisresearch. Withouthiscontacts,guidance,
mentorship,andfriendship,noneofthiswouldhavebeenpossible. Drs.RobertEmbley
andDanielFomariservedaschiefscientistsonmanyofthecruisestotheJuandeFuca
andEastPacificRiserespectively. Theirexampleofhowtobeaneffectiveseagoing
scientistmadealastingimpressiononme,andthisresearchbenefitedfrommany
discussionsofthestructureandvolcanicmorphologyofeasternPacificspreadingcenters.
ThecrewsoftheALVIN,AtlantisII,ROPOSandDiscovererdeservethanksfortheir
willingnesstopursuetheexcellencewerequired,evenintheweehoursofthemorning.
Dr.WilliamChadwickgraciouslypreparedthesamplelocationmapsinchapter2and
providedAMS-60datatoaidininterpretationofregionalgeologicassociations. H.Paul
Johnsongenerouslydonatedsplitsofbasaltsamplescollectedduringthreecruisestothe
CoAxialSegment. Dr.AnnHeatheringtonassistedgreatlyintheisotopicanalysesandI
thankher. Drs.TimO'HeamandW.I.RidleyanalyzeddatafromtheSmithsonianand
USGSrespectively,andDr.IanJonassonprovidedICPdatafromtheGSCatnocostto
thisproject. Iowethemalladebtofgratitude. Dr.KarenVonDamm,Dr.JohnLupton
andDr.MarvinLilleygraciouslyprovidedunpublisheddataforventfluidsdiscussedin
chapter3. D.JohnChadwickalsograciouslyprovidedhisunpublisheddataforbasalts
ii
fromAxialSeamountsouthriftzoneusedinchapter2discussions. Lastly,Iwouldlike
tothankRobinMilamforherassistanceintheformattingandpreparationofthis
manuscript,andmyfamilyfortheircontinualloveandsupport.
iii
11
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
ii
ABSTRACT vi
CHAPTERS
GENERALINTRODUCTION
1. 1
1.1 TheCoAxialSegment 7
1.2 TheEastPacificRisebetween9and10°N 8
2. GEOCHEMICALCHARACTERISTICSANDPETROGENETIC
RELATIONSHIPSOFRECENTLYERUPTEDMOREFROMTHE
CENTRALJUANDEFUCARIDGE: SEGMENTSCALE,
INTERFLOWANDINTRAFLOWVARIATIONS 11
2.1. Introduction 11
2.1.1 FieldInvestigationsandPreviousStudies 1
2.1.2 RegionalGeology 12
2.2. DataCollectionandSampleDistribution 16
2.2.1 AlvinandROVBasedSampleAcquisition 16
2.2.2 SurfaceBasedSampleAcquisition 16
2.2.3 SidescanSonarandNearBottomPhotographicDataAcquisition 20
2.3. AnalyticalMethods 20
2.3.1 SamplePreparation 20
2.3.2 MajorElementAnalyses 22
2.3.3 TraceElementAnalyses 26
2.3.4. IsotopicAnalyses 33
2.4. RegionalCoAxialSegmentGeochemicalCharacteristics 33
2.4.1 MajorElementTrends 33
2.4.2 IsotopicandTraceElementSignals 40
2.5.DiscriminationofDiscreteFlowUnits 46
2.5.1 VisualandBathymetricFlowDiscrimination 46
2.5.2 GeochemicalFlowDiscrimination 48
2.6. DistinctionofRegionalPetrochemicalProvinces 61
2.6.1 AxialSeamountProvince 62
2.6.2 TheWesternFaultBlockRidge 70
2.6.3 SmallNRZVolcanoes 7
iv
1
2.6.4TheCoAxialSegment 73
2.7. Discussion 74
2.7.1 RecentCoAxialSegmentVolcanism 74
2.7.2 Intra-flowGeochemicalVariability 78
2.7.3 RegionalPetrochemicalCharacteristics 81
2.7.4 RegionalMantleHeterogeneityandtheOriginsofCoAxial
SegmentGeochemicalDepletion 85
2.8.Conclusions 87
3. SUBMARINEINVESTIGATIONSOFATHIRD-ORDEROSCA9°37'N:
ESTABLISHINGACAUSEANDEFFECTRELATIONSHIPBETWEENOSC
PROPAGATIONANDMAGMATICACTIVITY 89
3.1. Introduction 89
3.2. PreviousStudiesandRegionalGeology 91
3.2.1 PreviousStudies 91
3.2.2 GeneralSecond-OrderScaleObservations 99
3.2.3 PreviousGeologicObservationsoftheEPRAxisBetween9°
49.9'Nand9°37.1'N 102
3.3. DataAcquisitionandAlvinObservations 104
3.3.1 InSituGeologicObservations 104
3.3.2 HydrothermalandBiologicActivity 109
3.4. BasaltGeochemistry 11
3.4.1 BasaltSampleRecoveryandAnalysisProcedures Ill
3.4.2 Second-OrderSegmentScaleGeochemicalObservations 118
3.4.3 Fourth-OrderSegmentScaleGeochemicalObservationsof
SegmentD 130
3.5. Discussion 131
3.5.1 SouthwardAgingofVolcanicActivity 131
3.5.2 The9°37'OSCasaMagmaticandHydrothermalDivide 133
3.5.3 EvidenceforaMagmaticPerturbationat9°37' 136
3.5.4 AssociationofE-TypeMORB 138
3.6. Conclusions 144
4. CONCLUSIONS 147
APPENDIX FREQUENTLYUSEDACRONYMSANDDESCRIPTIONSOF
SAMPLINGTECHNIQUES 151
REFERENCES
I53
BIOGRAPHICALSKETCH 164
V
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDoctorofPhilosophy
GEOCHEMISTRYOFEASTERNPACIFICMORE:
IMPLICATIONSFORMOREPETROGENESISANDTHENATUREOFCRUSTAL
ACCRETIONWITHINTHENEOVOLCANICZONEOFTWORECENTLYACTIVE
RIDGESEGMENTS
By
MatthewC.Smith
December1999
Chairman:MichaelR.Perfit
MajorDepartment:GeologicalSciences
Adetailedinvestigationoftworecentlyvolcanicallyactivemid-oceanridge
(MOR)segmentsintheN.E.PacificOceanhasresultedinabetterunderstandingof
crustalaccretionwithintheneovolcaniczonesofmedium-tofast-spreadingridges. The
useofsubmersibleandothernear-bottomsystems,inconjunctionwithrocksamplingby
waxcore,providesexcellentspatialcontrol,resultinginadatabaseofpreciselylocatedin
situgeologicobservationsandsamples. Thecombinationofdensehigh-resolution
sampling,temporalcontrolgainedbyrecenteruptiveactivity,andmulti-yearmonitoring
effortsmakestheseinvestigationsuniqueinMORresearch.
TheCoAxialSegmentoftheJuandeFucaRidge(JdFR)isamedium-spreading
rateridgethathasexperiencedatleastthreevolcaniceruptionsbetween1981and1993.
Evidenceofamagmaticextensionandchemicalheterogeneitybetweendifferentlava
flowsindicatesthattheCoAxialSegmenthasbehavedasamagmalimitedsystem,
despitetheoccurrenceofrecenteruptiveactivity. ThecurrentCoAxialSegment
neovolcaniczoneisgreaterthan1kmwide,andspatialfocussingoferuptiveactivityis
vi
poorrelativetothemoremagmaticallyrobustCleftSegmentoftheJdFRand9-10N
segmentoftheEastPacificRise(EPR). AbroadersurveyofthecentralJdFRreveals
that,onaregionalscale,geochemicallydifferentmeltregimescanbeassociatedwith
distinctstructural,morphologicandvolcanicprovinces.
Detailedstudyofridge-axismorphology,structure,andchemistryofthefaster
spreading,magmaticallyrobust,9°-10°NsegmentoftheEPRshowsadifferent
relationshipbetweenridge-axisstructureandmagmaticactivitythanthatobservedatthe
CoAxialSegment. Datafromathird-orderoverlappingspreadingcenter(OSC)located
at9°37'Nsuggeststhatthereisacloseassociationbetweenmagmaticactivityandaxial
discontinuities,andfurthersuggestthatthisOSCisamagmaticandhydrothermaldivide
betweenadjoiningthird-orderridgesegments. Temporalconstraintscorrelatearecent
magmaticeventtosouthwardpropagationoftheeasternOSClimb. Thesedataestablish
adirectcausalrelationshipbetweenmagmaticactivityandevolutionofridge-axis
discontinuities,andconfirmthatsegmentationofmeltdeliverytotheridge-axismay
affectridge-axisstructurealongthefastspreadingEPR.
vii
CHAPTER
1
GENERALINTRODUCTION
Themid-oceanridge(MOR)systemisaglobe-encirclingvolcanicmountain
chainmorethan60,000kmlong. Itextendsthroughoutalloftheworld'smajorocean
basins,risingsome1000-3000mabovetheabyssalseafloor. Mid-oceanridgesarethe
siteofoceaniccrustalaccretionandlithosphericgenesisand,assuch,areimportanttothe
studyofplatetectonicsandEarth'sgeologicalprocesses. Whileanunderstandingof
magmaticprocessesoccurringatMORsisanessentialpartofunderstandingplate
tectonics,thesignificanceofmagmaticactivityatoceanridgesextendsfarbeyond
geologicalinterests. MagmaticactivityalongMORsprovidesthesourceofheatthat
drivesextensivehydrothermalcirculationwithintheoceaniccrustwhich,inturn,exertsa
stronginfluenceonoceanchemistry. Notonlydoesthisvolcanicallydriven
hydrothermalsystemhelptocontrolseawaterchemistry,butitalsoprovidestheenergy
thatsupportsarichanddiverseabyssalecosystemthatutilizeschemosyntheticreduction
asthebasisofitsfoodweb.
Surprisingly,relativelylittleofthisimmensevolcanicsystemhasbeenstudiedin
detail. Difficultiesininvestigatingsucharemoteandextremeenvironmentlimiteddirect
examinationuntilthe1960'swhenadvancesintechnologyallowedmantotraveltoand
freelyaccessthemid-oceanridges,whichtypicallyrangefrom2000-4000metersbelow
sealevel. EarlystudiessampledtheMORsystemwithcoarsespatialresolution,
providinginsightsonsomeofthefirst-orderdifferencesbetweendifferentridges. The
1
2
adventofswathmappingsystemsthatallowforrapidunderwaybathymetricmappingof
largeareasofseafloorgreatlyadvancedtheunderstandingofMORmorphologyand
structure.
Discontinuitiesandoffsetsinthestrikeoftheridge-axisseparatetheMORsystem
intosegmentsofvaryingscale,dependingonthemagnitudeoftheaxialdiscontinuity.
Macdonaldetal.(1988)establishedfourdifferentordersofridgeaxisdiscontinuity
classifiedonthebasisofthediscontinuity'sspatialdimensions. Inthiscategorization,
whichistodaygenerallyrecognizedastheacceptednomenclature,first-orderspreading
segmentsaretectonicallydefinedandboundedbylargetransformoffsetsorpropagating
riftswhoseoffsetsaresuchthatplateboundariesbehaverigidly,juxtaposingcrustof
greaterthanapproximately0.5-1Myragedifferenceacrosstheoffsetboundary. Offsets
aretypicallygreaterthan50kmandpartitiontheridge-axisatintervalsof100-1000km.
Second-ordersegmentshavealengthscaleof50-300km,andtheyareboundedby
smaller,non-rigiddiscontinuitiessuchaslargeoverlappingspreadingcenters(OSC)with
anoffsetgreaterthan3-5kmorsmalloffset(lessthan20km)non-rigidtransformfaults.
Third-ordersegmentationdividestheridge-axisintocharacteristiclengthsof30-100km,
andthesesegmentsareboundedbysmallerOSCs(0.5to3-5km). ThesesmallerOSCs
areoftenassociatedwithsmall(lO'sofmeters)increasesinaxialdepths. Lastly,fourth-
ordersegmentshavetypicallengthscalesof10-50kmandarenotcommonlyassociated
withincreasedaxialdepthnearthesegmentends. Thesefourth-orderdiscontinuitiesare
definedbysmallnon-overlappingoffsetsoftheridgeaxisand/orsmall(r-5°)bendsin
theridge-axisreferredtoasdeviationsinaxiallinearity(DEVALS)[Langmuiretal.,
1986]. Althoughfirst-ordersegmentsandtheirassociatedaxialdiscontinuitiesare
3
thoughttobepersistentontheorderofmilhonsofyears,smallerridgeoffsetsmaynotbe
astemporallystable[Macdonaldetal.,1988b].
AsinvestigationsoftheMORsystemwereundertakenatfinerandfinerspatial
scales,itbecameapparentthatthemid-oceanridgeenvironmentwastheproductof
complexinteractionsbetweenmagmatic,tectonic,hydrologic,andbiologicprocesses.
Onlybycompilingdetailedinvestigationsofprocessesoccurringatdifferentspreading
segmentswouldamorecomprehensiveunderstandingofthisenvironmentbeachieved.
Todate,onlyasmallpercentageofthemid-oceanridgesystemhasbeensubjectedto
detailedmulti-disciplinaryinvestigationatthefirsttofourth-ordersegmentscale.
Oceanicridgesexhibitarangeofplatedivergenceor"spreadingrates". Full
divergenceratesvarybyoveranorderofmagnitude,andtoafirst-approximationridge
morphology,structure,andmagmaticactivityarestronglyinfluencedbythisspreading
rate[PerfitandChadwick,1998]. Typically,slow-spreadingridges(fullplate-divergence
rateontheorderof1-4cm/year),exemplifiedbyridgesintheAtlanticandIndianoceans,
risesteeplyfromtheabyssrelativetofaster-spreadingMORs. Theyaregenerallyrifted
byanaxialgrabenthatcanbelO'sofkilometersacrossandgreaterthan1500metersin
relief. Slow-spreadingridgeterrainsaregenerallymoregreatlyaffectedbyextensional
tectonicsandfaultingand,assuch,tendtobemuchrougherthanatfaster-spreading
ridges. Volcanicmorphologyoftheaxisfloor,whichisthesiteofactivecrustal
accretion,tendstobedominatedbydiscreteconstructionalfeaturesthatmaycoalesceto
formmoundsorridges. Lavamorphologyisoftenpillowed,acharacteristicassociated
withrelativelyslowlavaeffusiverates. Thezoneofcrustalaccretion,or"neovolcanic
zone,"isthoughttoberelativelywideandunfocussed(2-12kmwide),withrelatively